linux-kernel - Re: [PATCH 06/10] x86: reduce memory and stack usage in intel

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Date:	Wed, 26 Mar 2008 07:50:23 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Mike Travis <travis@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...e.de>
Subject: Re: [PATCH 06/10] x86: reduce memory and stack usage in
	intel_cacheinfo


* Mike Travis <travis@....com> wrote:

> * Change the following static arrays sized by NR_CPUS to
>   per_cpu data variables:
> 
> 	_cpuid4_info *cpuid4_info[NR_CPUS];
> 	_index_kobject *index_kobject[NR_CPUS];
> 	kobject * cache_kobject[NR_CPUS];
> 
> * Remove the local NR_CPUS array with a kmalloc'd region in
>   show_shared_cpu_map().

thanks Travis, i've applied this to x86.git.

one observation:

>  static ssize_t show_shared_cpu_map(struct _cpuid4_info *this_leaf, char *buf)
>  {
> -	char mask_str[NR_CPUS];
> -	cpumask_scnprintf(mask_str, NR_CPUS, this_leaf->shared_cpu_map);
> -	return sprintf(buf, "%s\n", mask_str);
> +	int n = 0;
> +	int len = cpumask_scnprintf_len(nr_cpu_ids);
> +	char *mask_str = kmalloc(len, GFP_KERNEL);
> +
> +	if (mask_str) {
> +		cpumask_scnprintf(mask_str, len, this_leaf->shared_cpu_map);
> +		n = sprintf(buf, "%s\n", mask_str);
> +		kfree(mask_str);
> +	}
> +	return n;

the other changes look good, but this one looks a bit ugly and complex. 
We basically want to sprintf shared_cpu_map into 'buf', but we do that 
by first allocating a temporary buffer, print a string into it, then 
print that string into another buffer ...

this very much smells like an API bug in cpumask_scnprintf() - why dont 
you create a cpumask_scnprintf_ptr() API that takes a pointer to a 
cpumask? Then this change would become a trivial and much more readable:

 -	char mask_str[NR_CPUS];
 -	cpumask_scnprintf(mask_str, NR_CPUS, this_leaf->shared_cpu_map);
 -	return sprintf(buf, "%s\n", mask_str);
 +	return cpumask_scnprintf_ptr(buf, NR_CPUS, &this_leaf->shared_cpu_map);

	Ingo
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